Urban microclimate and thermal comfort modelling: strategies for urban renovation

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International Journal of Sustainable Building Technology

and Urban Development

ISSN: 2093-761X (Print) 2093-7628 (Online) Journal homepage: http://www.tandfonline.com/loi/tsub20

Urban microclimate and thermal comfort

modelling: strategies for urban renovation

Irina Tumini, Ester Higueras García & Sergio Baereswyl Rada

To cite this article: Irina Tumini, Ester Higueras García & Sergio Baereswyl Rada (2016):

Urban microclimate and thermal comfort modelling: strategies for urban renovation, International Journal of Sustainable Building Technology and Urban Development, DOI: 10.1080/2093761X.2016.1152204

To link to this article: http://dx.doi.org/10.1080/2093761X.2016.1152204

Published online: 09 Mar 2016.

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Urban microclimate and thermal comfort modelling: strategies for urban

renovation

Irina Tuminia  , Ester Higueras Garcíab and Sergio Baereswyl Radaa

aplanning and urban design, university of bío-bío, concepción, chile; burbanism and territory government, technical university of madrid, madrid, Spain

ABSTRACT

The urban microclimate plays an important role in building energy consumption and thermal comfort in outdoor spaces. Nowadays, cities need to increase energy efficiency, reduce pollutant emissions and mitigate the evident lack of sustainability. In light of this, attention has focused on the bioclimatic concepts use in the urban development. However, the speculative unsustainability of the growth model highlights the need to redirect the construction sector towards urban renovation using a bioclimatic approach. The public space plays a key role in improving the quality of today’s cities, especially in terms of providing places for citizens to meet and socialize in adequate thermal conditions. Thermal comfort affects perception of the environment, so microclimate conditions can be decisive for the success or failure of outdoor urban spaces and the activities held in them. For these reasons, the main focus of this work is on the definition of bioclimatic strategies for existing urban spaces, based on morpho-typological components, urban microclimate conditions and comfort requirements for all kinds of citizens. Two case studies were selected in Madrid, in a social housing neighbourhood constructed in the 1970s based on Rational Architecture style. Several renovation scenarios were performed using a computer simulation process based in ENVI-met and diverse microclimate conditions were compared. In addition, thermal comfort evaluation was carried out using the Universal Thermal Climate Index (UTCI) in order to investigate the relationship between microclimate conditions and thermal comfort perception. This paper introduces the microclimate computer simulation process as a valuable support for decision-making for neighbourhood renovation projects in order to provide new and better solutions according to the thermal quality of public spaces and reducing energy consumption by creating and selecting better microclimate areas.

KEYWORDS

urban ; neighbourhood eco-efficiency; thermal comfort; outdoor; microclimate ARTICLE HISTORY received 12 august 2015 accepted 26 december 2015

1. Introduction

According to United Nations (UN) projections, the world’s urban population is expected to increase by 80% by 2050, from 3.3 billion in 2007 to 6.4 billion in 2050 [1]. The rapidly increasing concentration of people in urban areas combined with a focus on the liveability and vitality of cities has led to increasing interest in the quality of open urban spaces. It is well known that thermal sensation is an important factor for the use and perception of outdoor urban spaces; tak-ing weather conditions into account when designtak-ing cities can result in health, social, economic, and environmental benefits. Thus, by integrating social and environmental objectives, it is possible to improve the quality of life of cit-izens and revitalize obsolete residential areas, strengthening social interaction by improving the experiences that people have in outdoor spaces [1–6].

Urban areas are subject to undesirable thermal con-ditions due to changes in urban surfaces that alter the radiative exchange, humidity and thermodynamic pro-prieties of the environment. This causes a peculiar local urban microclimate, characterized by heat concentration in urban areas compared to surrounding rural areas, which causes alarming effects in many cities [3]. Urban areas without high climatic quality use more energy for cooling in summer and even more electricity for lighting. Moreover, high temperatures, wind tunnel effects in streets and unusual wind turbulence due to poorly designed high-rise buildings cause discomfort and inconvenience when attempting to engage in a range of outdoor activities [7].

Studies show that the use of outdoor spaces is influ-enced by several factors, thermal comfort among them; areas with a poor level of comfort may be avoided by

© 2016 taylor & francis

CONTACT Irina tumini irina.tumini@gmail.com

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microclimate, produces a number of signigicant negative effects:

• Environmental effects: the temperature enhance-ment causes an increase in the demand for cooling energy in warm climates, especially in the summer. Numerous studies show a direct relation between temperature increase in city centres and CO2 emissions and photo-oxidant gases concentration [7,13,14].

• Economic effects: The air-conditioning loads in buildings (residential and offices) create a large energy consumption as well as an increase in the costs related to the process and higher costs for more powerful mechanical equipment [7,13]. In addition, the incremental cost of supplying peak demand energy and the sanitary costs of air pollu-tion and temperature increase should be taken into account.

• Social effects: an increase in temperature in high-density urban areas may be an important risk factor for citizens, related to health and mortality. Different studies evidence that urban populations show greater sensitivity to heat effects compared to rural regions, specially amongst vulnerable sectors (children and the elderly) [15]. Increases in urban pollution cause specific diseases such as cardio-vascular disease, as well as asthma and other res-piratory problems, among others. Furthermore, outdoor spaces play a key role in recreation and socialization; therefore, unfavorable microclimate conditions could affect their usage.

Passive architecture is a good alternative for reducing energy consumption and improving the sustainability of built-up spaces. However, in order to achieve better results, it is necessary to introduce passive design concepts at the urban level by using bioclimatic urban design [47].

The literature on the urban climate focuses on under-standing the influence of different urban elements on microclimate formation with two main objectives: the forecasting of thermal performance and the design of palliative measures for urban spaces [16–19].

Besides considering differences with the surrounding rural areas, it is possible to notice and measure climatic differences between various zones within the same city [20,21]. Giridharan conducted extensive field measure-ments in order to identify the key urban design variables that influence the urban climate conditions in Hong Kong [21]. The research reveals that variables such as sky view factor, surface albedo, altitude, vegetation and the aver-age height-to-floor area ratio are crucial for mitigating the urban climate. Other variables such as wind velocity and solar radiation are also equally crucial, but designers people as a result [2,8]. Therefore, the amelioration of

microclimate conditions can contribute to outdoor envi-ronmental quality, providing spaces for mutual interac-tion between citizens and enhancing quality of life within cities. Moreover, there are economical and environmental advantages, i.e. reducing the energy demands on buildings for heating or cooling [9,10].

Several studies have focused on thermal comfort in outdoor urban spaces. In the Rediscovering the Urban Realm and Open Spaces (RUROS) project Nikolopoulou found a relationship between microclimate conditions and thermal comfort in a Mediterranean urban environ-ment [6]. In this work, the comfort conditions and peo-ple’s experiences and perceptions were evaluated using surveys, which consisted in microclimatic monitor-ing, structured interviews and observations of people’s behaviour in their natural environment. Other studies have focused on the significance of the links between human-biometeorology and town planning [11], such as the direct effect of the climate on people’s perception at a micro scale and the diversity of microclimates in urban areas [12]. This research strives to undestrand how the weather and climate affect people in outdoor urban environments.

Studies on urban microclimates have generally been oriented to defining countermeasures for new urban areas. Nevertheless, the renovation of deprived neighbourhoods will be the main challenge of architects and urban plan-ners in the twenty-first century, in order to bring the growth model back towards ‘urban development without growth’ – or, in other words, towards a more sustainable city model for existing cities. The aim of this paper is to introduce thermal comfort improvement as a target for the renovation of outdoor urban spaces through the use of microclimate computer simulation in order to support architects, urban planners and neighbours in the deci-sion-making process for urban renovation. The study of existing urban areas, and in particular the microcli-mate strategies that can be applied to the renovation of rationalist urban neighbourhoods, is the main focus of this research.

2. The microclimate in the urban environment 2.1. The influence of urbanization on the

microclimate

Urbanization produces different climate conditions from the surrounding rural areas, characterized by higher temperatures, diurnal temperature variation reduction, changes in wind direction and speed, different heat transference, and a peculiar rainfall balance. Regarding city sustainability, this phenomenon, known as the urban

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usually have less control over these in any built-up city. Therefore, the study reveals the strong interdependency between variables so that any intervention to mitigate microclimate effects should consider these variables within a comprehensive structure [21].

Several studies have focused on the use of vegetation as a form of improving microclimatic control in public spaces. Urban green structures can cool hot air by combin-ing shade and evapotranspiration effects, which result in a reduction in the radiant temperature and greater control over wind velocity and direction; furthermore, plants can regenerate air, absorb the dust that falls as temperatures decrease in the evening, and filter dust and noise [22].

Each urban microclimate is the consequence of several phenomena, including the regional climatic conditions, urban morphology and human activities. Therefore, various solutions are required in order to integrate all these aspect simultaneously. Due to the complexity of providing a comprehensive database for city features and the weaknesses of theoretical atmos-pheric models for urban environment on different scales, there is no single unique assessment method. Mathematical models have been developed to solve a variety of urban climate problems, using major sim-plifications due to the complexity of the urban envi-ronment. Nevertheless, computational techniques have advanced extensively over the past two decades, allow-ing researchers to dramatically improve the mathemat-ical models used to formulate solutions to large-scale problems. Among these models, energy balance and dynamical numerical approaches have resulted in the most reliable and satisfactory outcomes to date [14].

2.2. The nexus between microclimate and urban quality: thermal comfort

The thermal comfort of people in the outdoor environ-ment is one of the factors influencing outdoor activities in streets, plazas, playgrounds, urban parks, etc. The response to thermal comfort could be unconscious, but is often the result of the different use of urban spaces and its consequent increase or decrease in isolation and social exclusion. The success or failure of an urban space also depends on its climatic conditions [6,11,23].

In recent years, many models of thermal comfort have been studied with the purpose of finding direct and indi-rect links between human thermal sensation and the use of outdoor spaces. Several studies have shown the impor-tance of microclimate urban conditions in the perception of outdoor urban spaces, which can result in economic, social and health benefits if improved [24]. The design of comfortable spaces also anticipates knowledge of what is comfortable.

The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) defines comfort as ‘that state of mind which expresses satisfaction with the thermal environment’ [25]. Thus, the concept of thermal comfort is linked to the temperature sensation and the thermally neutral status.

A major group of biometereological models have been developed to describe thermal comfort in terms of physi-ological processes as well as how the heat-transfer proper-ties of the human body are linked to local microclimatic conditions [24]. The effector responses of metabolic heat production (basal + work) result in a heat loss or gain which affects the passive system in the new body temper-ature. The environmental parameters and heat production levels affect the relationship between those effectors and the body temperature [26].

Studies focusing on the physiological aspect are interested in how the thermal environment affects the regulation of the human body (sweating, shivering, vas-cular changes, etc.) using physiological models based on human-body balance. However, the heat fluxes between the environment and the human body are not easy to sim-ulate because, in addition to climatic parameters, other factors such as clothing and work activity come into play. Also, the relationship between global thermal sensation and changes in local conditions is not totally clear [27].

In the past 40 years, more complex multi-segmental models have been developed in order to simulate and predict the body’s physiological response to various con-ditions in greater detail. Following a comparison between different models, the Fiala multi-node thermo physiolog-ical model was selected to form the basis of the Universal Thermal Climate Index (UTCI) [28]. The UTCI was devel-oped as COST Action 730 of the International Society of Biometeorology with the aim of creating an interna-tionally-recognized comfort index for people in outdoor spaces based on a comprehensive physiological model which takes into account the component of adaptation.

The Fiala model can simulate the human body with a good degree of accuracy in terms of both the local and overall physiological response. The body heat losses are calculated taking into account the characteristic inhomo-geneity such as the non-uniformity of skin temperature, regulatory responses, clothing properties and environ-mental conditions [28]. The UTCI follows the concept of equivalent temperature (ET), which involves the refer-ence environment with 50% of relative humidity (RH, but vapour pressure capped at 20 hPa), calm air and radiant temperature making up the air temperature. The physi-ological response of exposure, taking into consideration clothing insulation, has been calculated for an individ-ual who is assumed to be walking at 4 km/h on ground level after 30 min and 120 min [28]. The UTCI defines

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in many cases, the outcomes are not applicable elsewhere [14,16].

The numerical approach is an alternative research method that many studies are utilizing due to two main reasons:

1) The numerical model is particularly suitable for highlighting the connection between the physical urban structure, the microclimate and thermal comfort by making the translation from the results to the practical design guidelines.

2) Compared to extensive field measurements, it is faster and less expensive, and it also allows com-parisons among numerous case studies and pro-ject scenarios [16,31–33].

Urban microclimate models differ substantially accord-ing to their physical basis and their temporal and spa-tial scale. The length scale for example can vary from a few metres to a few kilometres, and the timescale from a few seconds to seasonal variations lasting for a period of months. It is practically impossible to solve all the scales in a single model with the present available computational power. Despite this, the use of computers allows the solv-ing of differential equations numerically with appropriate boundary conditions and pressure and temperature pro-files on a predefined numerical grid [31]. On the micro scale, three-dimensional (3D) models include hydrother-mal processes and energy processes. Urban canyon mod-els are typical examples; they use simplified turbulence schemes which are combined with 3D flow modelling and 2D energy modelling. Furthermore, very few microcli-mate models evaluate thermal comfort, mainly due to the difficulty in determining the human body repercussions in urban areas produced by complex radiation fluxes. This problem is often caused by the use of simplified methods, in which many atmospheric processes are removed and replaced by data sets [16,31,34].

Several scholars have used ENVI-met to simulate the urban climate on the microscale. The software was developed by Michael Bruse at the University of Mainz, Germany in order to simulate the interaction between sur-faces, plants and air in an urban environment. The tool has been further developed to allow the analysis of the effect of small-scale changes in urban design (pattern, vegetation, building morphology, etc.) on the microclimate under mesoscale conditions defined at the beginning of the simulation (city climate, meteorological data, etc.) [35,37].

The main model is designed in 3D with two hori-zontal dimensions (x and y) and one vertical dimension (z). ENVI-met is based on thermodynamics and com-putational fluid dynamics (CFD) using non-hydrostatic incompressible Navier–Stokes equations (Equations (1a), (1b) and (1c)) with Boussinesq Approximation (Equation (2)):

the equivalent temperature for a given combination of air temperature, wind speed, humidity and radiation as the air temperature of the reference environment, which produce the same response. Figure 1 shows the assessment scale of the value of the UTCI ET [29].

For the evaluation of comfort in terms of external sys-tems based on the balance of heat needed for a reasonable comfort level for skin temperature and sweat rate, it can only be used in steady-state conditions [30]. A proper eval-uation must take into account the substantial differences between external and internal conditions such as exposure time, adaptation, metabolic activity and clothing. Among various existing comfort indices, the UTCI is the most suitable benchmark for comfort assessment because i) it can describe the well-being conditions of the individual, ii) it is easy to calculate using climatic parameters which can be obtained by simulation, and iii) it provides a wide range of results in order to distinguish between the diverse possible solutions.

2.3. An urban microclimate forecast using a numerical method

For the thermal behaviour analysis, the experimental methodological approach is probably one of the most widely used. However, works based on field measure-ments require a lot of specialized equipment and tech-nicians, rendering them expensive and time consuming. Furthermore, the theoretical formulation is very difficult due to the many factors involved in the microclimate and,

Figure 1. utcI assessment scale. Source: bröde, p, fiala d, et al. and (2012) deriving the operational procedure for the universal thermal climate Index (utcI). International Journal of biometeorology 56(3). p 483.

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design elements on microclimates and thermal comfort. The numerical thermal behaviour of each scenario is simulated using ENVI-met, and the results are compared in terms of thermal comfort performance using the UTCI.

Urban renovation projects are limited to passive strat-egies, given that the morphology, dimensions and ori-entation of the existing buildings cannot be modified. Therefore, this research project is restricted to the open spaces between buildings, where surface materials can be changed, green areas can be created, and shading systems can be established. For this reason, the analysis is limited to the comparison between two main strategies: a) the modification of green areas in terms of percentage and type, and b) the replacement of existing paving materials with cool materials for non-roof surfaces.

3.1. Case studies

In order to perform the simulation process, two case stud-ies located in Moratalaz, an open-block neighbourhood of Madrid, were selected. According to Köppen1 the climate in Madrid is defined as a Hot Steppe climate (BSh) char-acterized by low temperatures in the winter, hot summers and low precipitation [36]. The average temperatures in Madrid vary between 6°C in winter and 24°C in sum-mer. During the hottest months (July and August), daily maximum temperatures exceed 30°C, with the average humidity fluctuating between 39% and 41% and precipi-tation less than 15 mm per month [38].

Moratalaz is a residential neighbourhood located in the east, built in the 1970s through the public housing programmes implemented after the Spanish Civil War (1936–1939). In these social housing neigbourhoods, a novel composition of superblock lots with wide pedestrian spaces, green areas and tower blocks of different heights and configurations was created. The 1970s residential expansion areas are characterized by a very innovative urban organization model based on a rational structure: residential buildings on pilotis, tall blocks, large green areas between buildings and the creation of tree-lined boulevards in order to provide walkable spaces containing retail outlets and services [10]. These areas can be tested as a pilot action because there are plenty of other neigh-bourhoods built in the same way in Madrid which also require microclimate solutions.

The two case studies in this paper are:

• The Pavones neighbourhood, characterized by 5-floor buildings on pilotis of linear growing plants and tall buildings over 11 floors, NE-SW oriented at an angle of 27° azimuth south. The public space is occupied by car-parking areas, different types of gardens, vacant spaces and a tree-lined boulevard. To use a numerical model, the area of interest must be

divided into grid cells [33–35]. An extensive list of varia-bles provides a comprehensive description of the atmos-phere, surface and soil for each cell in the model (for more information, see the ‘scientific docs’ on the official website, http://www.envi-met.com).

Numerical simulation is commonly used by city planners to make decisions on parameters like building density and orientation for new urban areas [34]. The results of this work have shown that the numerical approach can help designers in the implementation of neighbourhood renovation projects too. Thus, different solutions can be evaluated with the aim of local microclimate amelioration. In the context of urban design, ENVI-met has been extensively used because of its capability of combining spatial variation in complex systems and thermal comfort [33]. In this paper, it is used to simu-late and compare conditions generated by modifying urban elements such as vegetation and surface materials, which are the main factors in outdoor space renovation.

3. Research methodology

The research hypothesis starts from the assumption that it is possible to define microclimatic behaviour patterns associated with urban morphology, and that microclimatic conditions affect thermal comfort in the urban environ-ment. Thus, it is possible to mitigate the worse impacts of the microclimate through modifying the existing urban public spaces, and consequently, improving thermal com-fort conditions for citizens.

The aim of this work is to explore the relations between neighbourhood renovation strategies, the microclimate and thermal comfort, thus contributing to a deeper understand-ing of microclimate processes and the capability to forecast them. The work is based on real case studies of different sce-narios for testing the effects of the modification of urban

(1a) 𝜕u 𝜕t +ui 𝜕u 𝜕x i = −𝜕p � 𝜕x +Km ( 𝜕2 u 𝜕x2 i ) +f ( v − vg ) −Su (1b) 𝜕v 𝜕t +ui 𝜕v 𝜕x i = − 𝜕p𝜕y +Km ( 𝜕2 v 𝜕x2 i ) +f(u − ug)−Sv (1c) 𝜕w 𝜕t +ui 𝜕w 𝜕x i = − 𝜕p𝜕z +Km ( 𝜕2 w 𝜕x2 i ) +g 𝜃(z) 𝜃 ref(z) −Sw (2) 𝜕u 𝜕x + 𝜕v 𝜕y+ 𝜕w 𝜕z = 0

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• The mesoscale conditions based on geographic location and local meteorological settings.

• The geometry of the model environment such as building morphology structure, plant and soil details.

• The simulation start date and time, the period to be simulated and the time interval for updating the model’s state (see Table 2).

The model domain is organized as a rectangular area which extends along the x-, y- and z-axes. The dimensions and resolution of this grid are user-defined in accordance with the objective of the simulation. The vertical grid can be equidistant or telescopic. In this research work, the real morphology of case studies was determined with an extension of about 2 hectares, and was overlapped onto a square grid of size dx=3 m, dy=3 m and dz=3 m, thus creating an equivalent model for the simulation.

Some of the important assumptions made in the ENVI-met simulation process are:

1. Flat terrain.

2. Box-shaped buildings.

3. A cubic grid with a maximum resolution of 1 m. 4. Empirical initial boundary conditions in order to

obtain good agreement with the average meas-urement data.

5. A constant wind profile during the simulation period.

6. A constant indoor temperature and no heat stor-age for buildings; therefore, it is not possible to take into account the emission of heat from buildings at night.

• The Fontarrón neighbourhood, characterized by 5-floor buildings in open blocks, positioned in a courtyard distribution within which are green private spaces, areas with sand, and isolated trees, NE-SW oriented at an angle of 48° azimuth south (Figures 2 and 3).

To describe the neighbourhoods, some of the indica-tors developed by Salvador Rueda for the Special Plan for Environmental Sustainability Indicators of urban devel-opment in Seville and the Victoria-Gasteiz Mobility Plan are taken as a reference (see Table 1) [39].

3.2. Simulation method

The simulations for the calculation of the microclimate changes produced by urban refurbishment scenarios were conducted in ENVI-met 3.1. The main advantages of this software are:

1. Simplicity of use and low demand of time in the use of software.

2. A good representation of the transfers between vegetation and soil surface with a multilayer configuration.

3. The possibility of using a small horizontal and vertical grid with a precision of up to 1 m.

4. The low number of input parameters for the whole vegetation–soil–atmosphere system

[16,33,37,41,42]The procedure used to build the simulation models is detailed below. The software allows the user to define:

Figure 2. Image of the case study areas in moratalaz, madrid.

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temperature is not relevant to this simulation because it is based on a comparison between the current state and project scenarios, in which neighbourhood renovation will only involve outdoor spaces (green areas and sur-faces materials).

The simulations were performed using the typical ther-mal conditions of summer and winter, simulating 24-h results every 180 min. In order to set the initial climate data, the values recorded on 15 August 2011 (summer con-ditions) and 21 December 2011 (winter concon-ditions) from the Madrid-Retiro meteorological station of the National 7. A 1D soil model with a 5-level profile of humidity

and temperature.

8. A vegetation model considering the interaction of humidity and radiation in the soil and the air. The Ag-s model [37] calculates the photosynthe-sis rate of the plants and from this determines the CO2 demands and finally the state of the stomata [44].

Since the aim is to evaluate the level of thermal com-fort during the daytime, the constant wind profile is valid for the case studies presented. The constant indoor

Figure 3. Images of locations from the cases studies as follows: a) pavones parking area, b) pavones boulevard, c) fontarrón parking area 1, d) fontarrón parking area 2, e) pavones urban canyon, and f) fontarrón urban canyon.

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comparable to one another, and secondly due to the need to mitigate the maximum conditions of thermal discomfort [14,27]. Furthermore, several studies showed the direct rela-tionship between elevated mortality levels and high temper-atures, especially regarding people over 65 years old. Miron et al. studied the time trend for the maximum temperature of the minimum organic-cause mortality in Castilla-La Mancha (Central Spain) for the period 1975 to 2003, and ascertained that the increase in mortality is attributable to the decrease in comfortable temperatures [45]. Also, according to the pro-jections of global warming due to climate change, summer Spanish Agency of Meteorology (AEmet) was used (see

Table 3). According to the data available (Madrid-Retiro 2011) , the simulation days are selected because they are representative of summer and winter conditions.

This initial study was essential in order to identify the most critical hours during the day with respect to thermal comfort conditions, which correspond to 3:00 p.m. in the summer. The most critical condition is used for the evalua-tion and comparison of numerous hypothetical project sce-narios (see Table 4). This selection was made firstly because of the need to obtain a hypothesis results that are easily

Table 1. details of the indicators selected for this study from the 84 indicators in [39,40].

notes: 1Surface filtration coefficient: • Impervious soil = 0.00 • partially impervious soil = 0.30 • Semi-impervious soil= 0.50

• green area without connection with natural soil = 0.50 • green area with connection to natural soil = 0.70 • green area on natural soil = 1.00

Source: S. rueda, “Special planning of environmental Sustainability Indicators for urban activities in Seville", gerencia de urbanismo. ayuntamiento de Sevilla, barcelona, 2006.

 Indicator description, objective and units Reference Values Pavones Fontarrón

1.Building Density: defined as the number of dwellings per hectare. the main

purpose of this indicator is to control the density of construction, in order to avoid a diffuse city spread and congestion problems due to very high density. building density d (n/ha)= dwelling (n)/ total area (ha)

60.00-200.00 66.67* 142.58

2. Absolute Compactness: defined as a building volume per square meter of

urban area. the value indicates the pressure of buildings in the grid of the city. this element gives a guide for the proximity of buildings to each and the potential to condense a multi-function area into a limited space. abso-lute compactness c (m3/m2)= building volume (m3)/urban area (m2)

5.00-7.50 3.94 3.33

3. Permeability index: the aim of this indicator is to reduce impervious soil

and to promote the natural water cycle. the filtration capacity is defined according to a surface filtration coefficient related to the soil type1. the value is defined as Ip (% of m2/m2)= ∑ (area x coefficient)/total area.

0.30 0.27 0.23

4. Percentage of roads: this indicator shows the impact of cars on public

spac-es, including alternative transportation systems (bicycles and pedestrians). percentage of roads (%)= ∑road for motorized(sm)/total road(sm).

25% 57% 37%

5. Number of trees per built square meter: the intent of this indicator is to

en-sure a minimum number of trees are situated in the urban area, according to the urban grid. this value should change with the urban typology: trees per m2: nº of trees/ area (m2).

1t/20m2 0.09 0.15

Table 2. the primary input parameters used in the envI-met simulation.

*default envI-met value used.

Type Parameter Value

air temperature Initial temperature atmosphere (K) 300

Wind Wind speed in 10 m ab. ground (m/s) 2.24

Wind direction 21

roughness length at reference point 0.10*

humidity Specific humidity in 2500 m (g water/kg air) 8.89

  relative humidity in 2 m (%) 41

location location madrid

latitude (°) 40.23*

longitude (°) −3.43*

Wall thermal resistance ([m2 K]/W) 0.8*

albedo 0.4*

roof thermal resistance ([m2 K]/W) 1.0*

  albedo 0.3*

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Other systems are based on the use of pervious mate-rials in which evaporation contributes to temperature decreases [47].

Unquestionably the most effective measure for reduc-ing unfavourable microclimate conditions however is the use of green areas and trees. The use of natural soil and trees has both direct and indirect effects on the urban microclimate. The direct effects include the creation of shade and wind protection, whilst the main indirect effect is the evapotranspiration of the plants [2,14,21,48].

3.3. Simulated scenarios

Different renovation proposals were tested using the two case studies (Pavones and Fontarrón) and the results deliv-ered by the project scenario simulations were compared to the current state. For the selection of surface materials, the scenarios are as follows:

mortality will increase substantially while winter mortality will decrease [45,46].

This research focuses on the thermal behaviour in the interstitial space between buildings. Thus two main aspects were focused on: surface paving materials and green areas.

An initial approach that can be used to reduce the neg-ative effects of a microclimate is to work on the horizontal surface materials: the streets, squares, parking areas and even roofs that cover a significant percentage of urban surfaces.

In recent years, several studies have investigated the use of cool materials. Cool pavements refer to surface materials that tend to store less heat and have a lower surface temperature compared with traditional materials. Different strategies are available for implementing cool pavements. The primary methods are the use of highly reflective materials that reduce heat absorption or com-posite structures that emit a lower level of heat at night.

Table 3. Weather data collected in the madid-retiro meteorological station.

note: rh = relative humidity; ta = air temperature; W = wind speed.

Source: data provided by aemet (http://www.aemet.es/es/datos_abiertos/catalogo).

21 December 2011 15 August 2011 Time Ta (°C) RH (%) W (m/s) Ta (°C) RH (%) W (m/s) 0 3.91 50 1.7 24.96 26 1.2 3 3.96 71 1.6 22.17 35 1.1 6 4.02 68 1.7 19.65 42 0.9 9 2.41 59 1.6 19.03 45 0.8 12 5.38 66 1.9 23.56 35 1.2 15 7.39 45 1.8 27.62 24 2.1 18 6.56 45 1.6 29.93 18 2.3 21 4.76 53 1.7 28.94 19 1.8

Table 4. results for the thermal comfort assessment of the two case studies for summer conditions.

note: rh = relative humidity; ta = air temperature; tmrt = mean radiant temperature; utcI = universal thermal climate Index; W = wind speed. the worst thermal comfort conditions occurred at 3 p.m. in both cases.

Time

Parking area Urban canyon

Ta (°C) Tmrt (°C) RH (%) W (m/s) UTCI (Ceq) Ta (°C) Tmrt (°C) RH (%) W (m/s) UTCI (Ceq) Pavones 12:00 a.m. 21.18 15.27 86 1.40 20.40 21.48 15.06 96 1.49 21.10 3:00 a.m. 20.48 13.95 86 1.15 19.60 20.21 13.90 97 1.44 19.70 6:00 a.m. 19.78 13.27 86 1.13 19.10 19.23 13.07 98 1.42 18.50 9:00 a.m. 21.39 60.62 80 0.88 33.60 21.51 20.80 97 1.31 23.20 12:00 p.m 29.35 67.70 62 0.96 40.10 29.65 67.08 75 1.30 41.00 3:00 p.m. 30.85 70.27 60 1.13 41.70 31.26 62.73 73 1.47 41.60 6:00 p.m. 28.10 63.36 68 1.25 38.20 28.00 27.00 85 1.57 30.70 9:00 p.m. 23.60 17.38 77 1.20 22.90 23.65 17.08 93 1.55 23.80 Fontarrón 12:00 a.m. 23.52 15.00 66 0.54 22.00 22.90 15.03 70 1.47 20.70 3:00 a.m. 22.29 14.12 70 1.30 20.50 22.99 14.22 66 1.45 20.30 6:00 a.m. 21.91 22.62 66 0.50 22.90 21.91 13.52 70 1.40 19.50 9:00 a.m. 23.15 64.99 59 0.70 34.90 23.04 20.60 64 1.20 22.40 12:00 p.m 26.42 72.77 61 0.60 39.20 28.50 67.00 59 1.30 38.70 3:00 p.m. 27.60 75.10 61 0.60 40.06 29.87 69.51 57 1.46 40.30 6:00 p.m. 26.80 52.42 62 0.65 34.50 27.71 51.59 63 1.55 34.30 9:00 p.m. 24.51 17.51 64 0.50 23.40 24.11 16.90 67 1.50 22.20

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examined were a square with a parking area and urban canyons. One parking area was analysed in Pavones (see Figure 5(a)), and two in Fontarrón (Fontarrón 1 and 2, see Figure 5(b)). The urban canyons selected were two pedestrian streets with a height to canyon width (H/W) ratio of 1 in Fontarrón (see Figure 5(b)), and a parking area in Pavones with a H/W ratio of 0.5 (see Figure 5(a)). The aim was to check the impact of the renovation projects and identify the optimal strat-egies for rationalist urban typology.

The current state simulations (Table 5) show that the parking areas, which use black finishing material (asphalt), are where heat storage occurs, and the pro-cess is more intense during the summer months. The results shows that the system is sensitive to soil type, and thus the tool can be used to support the selec-tion of materials. The main effect of urban geometry is in heat distribution and dissipation: heat dissipation occurs at a low height in the parking area, while in the urban canyon the dissipation depends on many factors, such as the H/W ratio, wind speed, façade orientation and exposure. Green areas, especially with tall trees, are those where lower temperatures, higher humidity and milder winds are recorded. The mitigating effect of vegetation is more relevant in the summer. This result also seems coherent with the actual changes that take place in outdoor urban spaces, as in the winter the effects of shading and evapotranspiration produced by trees, which are mainly deciduous, are less than in the summer.

An analysis of the various design scenarios was con-ducted to compare the thermal conditions of the current state with the results obtained from each simulation. The diverse results demonstrate a complexity of variables and processes involved in the microclimate of the urban space. The comparison shows that an increase in green areas produces an air temperature reduction and better thermal (1) Replacing asphalt soil with white asphalt for

roads and semi-pervious soil with turf block for parking places.2

(2) Replacing asphalt soil with Perfect Cool for roads and semi-pervious soils with turf block for parking places.3

(3) Reducing green areas and pervious soil by replacing them with traditional materials (asphalt and concrete).

And for the green areas, the scenarios are:

(4) Improving the green areas using tall trees (20 m tall with a leaf area density of 0.3) for up to 30% of the total surface of the open areas.

(5) Improving the green areas by using shrubs (1.5 m tall) for up to 30% of the total surface of the open areas.

(6) Replacing impervious soil with grass.

The analysis was conducted by comparing the parameters of weather – air temperature (Ta), mean radiant tempera-ture (Tmrt), relative humidity (RH%) and wind speed (W) – and thermal comfort (UTCI) for the actual conditions and the six scenarios described above. Conditions from 9:00 a.m. to 9:00 p.m. are taken into account to define the most unfavourable conditions.

4. Results

The analysis of the thermal conditions for the six project scenarios compared the scenario simulation outputs with the current state for the air temperature (Ta, °C), mean radiant temperature (Tmrt, °C) and UTCI ( Ceq). The vertical definition of all the model domains considers a 0.4 m vertical grid size in the five lowest grid cells. The section at 1.2 m from the ground (grid 3z) was chosen because it represents the ther-mal conditions perceived by users. The outdoor spaces

Figure 4. envI-met 3.1 simulation model of a) pavones and b) fontarrón. Source: author elaborate using.

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though the variations from the current state are lower, espe-cially in the Fontarrón case studies In addition, scenario 3 (reducing the green area) causes an increase in thermal stress in Pavones (P.3) but an amelioration in Fontarrón (F1.3 and F2.3). The use of white asphalt and turf blocks in Fontarrón (F1.1 and F2.1) causes a reduction in Ta and an increase in Tmrt and thermal stress.

Only scenario (4), which increases the green area with tall trees, produces an air temperature reduction and comfort conditions, while changes in the soil cause an

air temperature reduction but worsening thermal comfort conditions.

According to the results in the parking areas, the substitu-tion of asphalt with Perfect Cool (scenario 2) results in worse thermal conditions than the current state (ΔUTCI=2.8), while the white asphalt (scenario 1) results in an enhancement of discomfort sensation equal to 0.6 in Fontarrón 1 and 1.1 in Fontarrón 2. The urban canyon behaviour is similar, even

Figure 5. aerial view and stereographic diagrams for (a) pavones (b) and fontarrón. Source: googlemaps and authors (using ecotect 2010).

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microclimate conditions; thus it is well suited to com-parative analyses such as this one. The comparison of the six scenarios and the wide diversity of results shows that urban microclimate forecast is complex and requires a case-by-case analysis. The numerical approach may be useful and efficient in guiding designers in the deci-sion-making phase to choose the most suitable solutions for renovation urban projects.

In terms of the renovation strategies proposed, increas-ing green areas produces a thermal discomfort mitigation effect due to the simultaneous occurrences of shade and evapotranspiration. The green areas improve the micro-climate in both open areas (parking) and urban canyons. The rationalist urban typology in particular, with its high density and wide open areas, offers the opportunity to provide large green areas with tall trees. The reduction of spaces dedicated to car movement and parking is another strategy that can be used to improve the green area per-centage in existing urban spaces.

Regarding the use of cool materials, the compar-ison shows two opposite effects at the same time: a reduction in Ta and an increment in the UTCI (see Table 5 and Figure 6). According to the results of [51], results in thermal discomfort mitigation. The green area

increase using tall trees is the best solution in terms of the reduction of Ta and UTCI difference, and thermal com-fort amelioration. In the Pavones urban canyon (P.4) the UTCI reduction of 9.2°C is achieved, and in the Fontarrón parking (F2.4) it is greater than 10°C, passing from ‘very strong heat stress’ to ‘moderate heat stress’ (see Figure 6).

5. Discussion

The result of the climate data and UTCI analysis demonstrates that there is not a direct relationship between air temperature and thermal comfort perception, but that many factors are involved. The results of the thermal comfort analysis for the summer period show that the primary factor in Tmrt, which depends on multiplicity radiations and the ability to emit and absorb heat. In the urban environment, the radiation energy absorbed by pedestrians (which results in thermal perception) is derived mainly from a long wave domain that depends on the temperatures of surfaces and reflected waves, among other parameters [50].

The UTCI, thanks to its wide range of results, enables differentiation between even slight deviations in

Table 5. Simulation results compaing the current state with the conditions projected for each scenario.

note: rh = relative humidity; ta = air temperature; tmrt = mean radiant temperature; utcI = universal thermal climate Index; W = wind speed.

Number Scenario Ta (°C) Tmrt (°C) RH (%) W (m/s) UTCI (Ceq)

Parking area Pavones pc current state 31.26 62.73 72 1.47 39.70 p2 perfect cool 29.94 55.65 63 1.10 42.50 p3 green elimination 29.79 69.41 59 0.91 40.70 p5 Shrubs 30% 30.34 70.80 63 0.88 41.90 p4 trees 30% 29.11 54.75 67 0.94 37.20

p6 perfect cool grass 27.70 60.20 75 0.76 37.30

Fontarrón f1c current state 29.87 69.51 58 1.46 40.30 f11 White asphalt 27.66 78.00 61 1.26 40.90 f13 green elimination 28.83 68.62 56 1.20 39.30 f15 Shrubs 30% 28.52 68.45 58 1.18 39.20 f14 trees 30% 25.94 59.34 73 1.28 35.80 Fontarrón 2 f2c current state 29.60 69.39 66 1.59 40.50 f21 White asphalt 27.96 78.47 69 1.43 41.60 f23 green elimination 28.83 68.62 56 1.20 39.30 f25 Shrubs 30% 28.26 59.35 68 1.15 37.40 f24 trees 30% 26.72 32.33 77 1.38 29.80 Urban canyon Pavones pc current state 31.26 62.73 72 1.46 42.00 p3 green elimination 30.78 69.07 68 1.49 41.80 p6 grass 29.45 54.28 85 0.98 39.10 p5 Shrubs 30% 30.49 54.00 79 1.03 39.50 p4 trees 30% 29.08 31.06 82 1.15 32.80 Fontarrón fc current state 27.70 62.34 58 0.27 37.70 f1 White asphalt 26.65 61.93 60 0.42 36.70 f3 green elimination 27.14 67.91 56 0.43 38.20 f5 Shrubs 30% 26.70 61.94 59 0.42 36.70 f4 trees 30% 26.21 61.76 61 0.42 36.40

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shading rather than changing the surface materials that are used.

6. Conclusions

This paper presents the results of a thermal comfort analy-sis carried out in order to investigate the effects of different the simulation results show that the increase in ground

albedo has the smallest influence on the microclimate, whereas the urban geometry is much more important. Finally, in terms of thermal comfort, the unfavourable outcomes obtained using cool materials suggest that renovation strategies in urban areas should be oriented towards improving green areas or providing better

Figure 6 results of the case study for parking areas and urban canyons: comparison of air temperature (ta), utcI and mean radiant temperature (tmrt).

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Notes

1. The Köppen climate classification was developed by Russian and German scientist Wladimir Köppen in 1884, and is one of the most widely used climate classification systems.

2. White asphalt is a cool material used for the construction of roads and pavements. The material characteristics used for the simulation were a solar reflectance 0.55 and an emissivity value of 0.9 [47].

3. PerfectCool is a dark colored pavement coating with high albedo. Laboratory tests have revealed that PerfectCool can reflect up to 81% of near infrared waves, has a low heat conductivity of 0.252 W/mK, has a high albedo of 0.46 and an emissivity value of 0.828 [49].

ORCID

Irina Tumini   http://orcid.org/0000-0001-8306-5384 References

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